The goal of scaling lasers to high output powers by phase locking multiple low-power modules has motivated the search for improved laser coupling techniques. Conventional methods for phase locking have relied upon optical coupling to induce cooperative lasing at a frequency and phase that is common to all lasers in the system. The strength of coupling is generally enhanced through optical mode matching wherein the output mode of one resonator is imaged onto the spatial mode of another resonator. Cavity mode matching can be achieved using conventional optical techniques (mirrors and lenses), but is prohibitively complex for a large number of lasers with diverse spatial mode characteristics. Optical phase conjugation can be used to generate automatic mode matching and greatly simplify the task of coupling cavities. The photorefractive process known as mutually incoherent beam coupling (MIBC) is particularly well suited to this task since the photorefractive grating that leads to coupling can be generated by mutually incoherent inputs. Thus, coupling is established without requiring the lasers to be phase locked a priori.
If the coupling paths are long and prone to mechanical instabilities, time-dependent variations in the optical path lengths can lead to relative phase fluctuations in the resonator outputs and degrade the degree of phase locking. One method for stabilizing the optical path lengths and further simplifying the alignment of the coupling beams is to use optical fibers to propagate the beam to the photorefractive crystal. One problem that arises in laser coupling via optical fibers is that of wavefront distortion due to modal diffusion and dispersion in the multimode fibers. Upon entering a multimode fiber, an input wavefront excites the corresponding propagation modes of the fiber. Scattering from impurities and structural defects causes diffusion of light into additional propagation modes. The different modes propagate with different group velocities causing severe wavefront aberrations that reduce the mode-coupling efficiency in a phase-locking setup. Modal dispersion can be avoided with single-mode optical fibers, but mode matching the laser to the fiber adds complexity to the alignment. Furthermore, a laser with a non-Gaussian output profile will not couple efficiently into the single-mode fiber. The problem of modal dispersion is effectively eliminated in multimode gradient-index fibers. Unfortunately, dissimilar propagation characteristics for meridional and skew rays lead to aberrations in the transmitted wavefront. A second source of reduced laser coupling efficiency is polarization scrambling. A polarized input beam will experience gradual depolarization as it propagates in the optical fiber. For the case of polarized laser devices, depolarization of the coupling light can introduce as much as a 50% decrease in the coupling efficiency.
Previous experiments indicate that phase conjugation in photorefractive barium titanate can compensate for wavefront distortions induced by modal dispersion. In these experiments, an image bearing beam is injected into a multimode optical fiber. The transmitted wavefront, severely distorted due to modal dispersion, is phase conjugated and retro-propagated through the same fiber. After double passing the fiber the original wavefront is reconstructed. Further studies of the two-pass fiber geometry show that phase conjugating a single polarization component of the fiber output leads to reconstruction of the input polarization as well.